Hydrocarbon cracking

ABSTRACT

STRACTED FROM THE FIRST PORTION OF THE HYDROCARBON CRACKED IN THE FIRST PORTION OF THE ZONE.   THE TENDENCY OF HYDROCARBONS TO FORM HIGHER MOLECULAR WEIGHT DERIVATIVES THEREOF DURING CRACKING IS DIMINISHED BY CRACKING A HYDROCARBON LESS SUSCEPTIBLE TO THE FORMATION OF HIGHER MOLECULAR WEIGHT DERIVATIVES THEREOF IN A FIRST PORTION OF A CRACKING ZONE AND INTRODUCING TO THE CRACKING ZONE DOWNSTREAM OF THE FIRST PORTION THEREOF AN ADDITIONAL HYDROCARBON FRACTION MORE SUSCEPTIBLE TO THE FORMATION OF HIGHER MOLECULAR WEIGHT DERIVATIVES AND CRACKING THE LATTER IN THE PRESENCE OF HYDROGEN AB-

C. S. KELLEY HYDROCARBON CRACKING Feb. 23, 1,971

Filed May 26. 1969 mZmDJOP INVENTOR.

C. S. KELLEY ATTORNEYS Patented Feb. 23, 1971 3,565,970 HYDROCARBON CRACKING Carl S. Kelley, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed May Z6, 1969, Ser. No. 827,659 Int. Cl. C07c 3/08; C10g 9/36 U.S. Cl. 260-683 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the cracking of hydrocarbons under conditions whereby the production of undesirable products is minimized. In accordance with another aspect, this invention relates to a process for cracking higher molecular weight hydrocarbons in the presence of partially cracked lighter molecular weight hydrocarbons to produce a desirable mixture of oleiins. In accordance with a further aspect, this invention relates to a process for the cracking of light and heavier hydrocarbons with a minimum production of dienes and polymers by cracking the heavier hydrocarbons in the presence of partially cracked lighter hydrocarbons. In accordance with a further aspect, this invention relates to the cracking of paraffinic and olefinic hydrocarbons with a minimum production of dienes by cracking the olefinic hydrocarbons in the presence of partially cracked parafinic hydrocarbons. In accordance with a further aspect, amylenes and ethane produced in naphtha cracking are separated and charged to further cracking to produce propylene and ethylene with a minimum production of dienes by injecting amylenes into the ethane cracking zone after partial conversion of ethane to ethylene and hydrogen.

The improvement of operation and control of thermal and catalytic hydrocarbon cracking units and operations continues to be of considerable interest due to the dependency of manufacturers on these modes of hydrocarbon conversion for upgrading considerable volumes of raw materials and intermediates. A great variety of productscan be produced by cracking a given hydrocarbon feedstock by either thermal or catalytic methods, or combinations thereof, depending upon the type and sequence of process steps involved, the catalyst employed, operating conditions, such as temperatures, pressures and contact times, and many other process variables. In most instances, a great variety of products are produced including many which are undesirable in specific applications or market areas. The selectivity of any given process for the desired primary products and, consequently, the elimination of undesired by products can be enhanced by prudent control of the relevant process variables; however, in many instances the compromise which must be made to promote a high degree of selectivity to the desired primary products detracts from the value of the overall process in other respects such as decreased overall conversion and production rate. This latter difiiculty is one of considerable magnitude in the thermal and catalytic conversion of relatively high molecular weight hydrocarbons such as amylenes which undergo hydrogen abstraction, i.e., conversion to lighter olefins. These olens are known to condense and polymerize with other reactants in the process with the resultant formation of higher molecular weight derivatives approaching the consistency of tarry carbonaceous residues.

I have found that the degree of this problem can be substantially reduced in any particular instance while at the same time accomplishing the conversion of hydrocarbons less susceptible to polymer formation by reacting these two varieties of hydrocarbons simultaneously in the same facilities in a preferred sequence hereinafter detailed.

It is therefore one object of this invention to provide an improved hydrocarbon cracking process. It is yet another object of this invention to provide a method for reducing the amounts of higher molecular weight derivatives produced in hydrocarbon cracking. It is yet another object of this invention to provide a method for cracking two dissimilar hydrocarbon fractions to produce desired products from each while minimizing the production of undesirable by-products.

SUMMARY OF THE INVENTION In accordance with this invention, a first hydrocarbon or mixture of hydrocarbons having a tendency to form ole- -tins and higher molecular weight derivatives thereof under cracking conditions is cracked in a second portion of a cracking zone in the presence of reaction products produced by cracking a second hydrocarbon or mixture of hydrocarbons less susceptible to the formation of higher molecular weight derivatives thereof in a first portion ofl said cracking zone upstream of said second portion. I have found that by this procedure the reaction products produced in the first portion of the cracking zone, particular hydrogen abstracted from the hydrocarbon introduced to the first zone, reduce the degree to which higher molecular weight by-products are produced in the second portion of the cracking zone.

In accordance with one embodiment of the invention oletins are cracked in the presence of partially cracked paraffins to produce lighter olefins.

In accordance with another embodiment of the invention, amylenes are cracked in the presence of partially cracked ethane to produce ethylene and propylene.

In accordance with a further embodiment of the invention, amylenes and ethane produced in naphtha cracking are separated and charged to further cracking to produce propylene and ethylene with a minimum production of dienes by cracking amylenes in the presence of partially cracked ethane to produce the ethylene and propylene.

In a preferred embodiment, the amylenes are injected into the ethane cracking zone after partial conversion of ethane to ethylene and hydrogen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This method is applicable to both thermal and catalytic cracking systems employing all varieties of hydrocarbon feedstocks, however, it is particularly advantageous in thermal cracking operations employing relatively high molecular weight feedstocks which otherwise are converted in substantial amounts to higher molecular weight derivatives under conditions suiciently severe to promote economically feasible conversion levels.

The advantages of this invention are particularly applicable to hydrocarbon refining facilities employing feedstocks having a wide range of characteristics such as the naphtha cracking facility described in U.S. Pat, 3,384,- 570. Any portion of the products produced by the process described in that patent comprise a predominantly amylenes fraction and an ethane fraction which can be 3 separated from products falling intermediate these two components by fractionation. These two fractions are suitable for conversion in the sequential cracking facility provided by the concept of this invention.

This procedure is preferred in many instances due to the fact that the ethane is upgraded to ethylene while at the same time the amylenes fraction can be converted to more desirable lower molecular weight products such as ethane, ethylene, butenes, butane, propylene, etc. However, I have found that the thermal cracking of amylenes alone results in the production of substantial amounts of higher molecular weight derivatives, e.g., polymers which result in reactor fouling by carbonaceous buildup and reduced selectivity and overall conversion levels. The degree .of this problem is substantially reduced by cracking the ethane recovered from the process of Pat. 3,3 84,- 570 in a first portion of a cracking zone or coil to convert at least a portion of the same to hydrogen and ethylene and injecting the amylenes fraction at an intermediate point in the cracking zone, i.e., at 4the beginning of the second portion of the zone, so that the C5 fraction will be cracked in the second portion of the zone in the presence of the reaction products produced in the rst portion of the cracking coil.

Selectivity to the desired products is substantially improved by this procedure while production of undesirable products and car-bonaceous residues is substantially reduced by this procedure.

Although an innumerable variety of hydrocarbons can be employed in the concept of this invention, it is presently preferred -that the hydrocarbon cracked in the rst portion of the cracking coil have from abou-t 2 to about 5 carbon atoms. It is also presently preferred that the feedstock to the initial portion of the cracking zone be substantially saturated since by this procedure higher hydrogen concentrations can be realized in the downstream, i.e., second, portion of the cracking coil. It is presently preferred that the hydrocarbons cracked in the second portion of the cracking coil have from about 3 to about `8 carbon atoms, vand be normal or branched chain oletins. Operating temperatures can yalso vary considerably and will, of course, depend on the nature of the reactants to be cracked and the product distribution desired of and the degree of conversion and selectivity desired. However, operating temperatures generally will be within the range of from about 1400 to about 1500 F. for the rst portion of the cracking zone `and from about 1450 to about 1650" F. for the second portion of the cracking zone. Substantial variation in overall performance, conversion, selectivity, etc. can also tbe realized by varying the overall residence time, which will usually be within the range of from about 0.2 to about 3.5 seconds.

Although notice-able improvement is obtained at all ranges o'f contact time and feed rates, substantial advantages are derived when, under these conditions, the residence time in the rst portion of the cracking zone is preferably within the range of from about 0.1 to about 3 seconds, While the residence times in the second portion of the cracking zone are generally in the range of from about 0.1 to about 0.3 seconds, and the ratio of the mass of hydrocarbon injected into the first portion cracking stage to the mass of hydrocarbon injected into the process stream in the second stage will generally be within the range of about .1 to about 5.

DESCRIPTION OF THE DRAWING' The invention can be better understood by reference to the drawing, which is intended only to be a schematic illus- `trat-ion of one embodiment of the concept of this invention. The description includes some operating conditions which can be employed.

A naphtha stream comprising primarily C5 hydrocarbons is passed by way of line 10 to cracking zone 11. The naphtha can be a broad range naphtha stream which has not been previously fractiona-ted or it can be a heart Cil cut of C5 hydrocarbons recovered by fractionation (not shown). Steam is introduced into the cracking zone by way of line 12. Cracking zone 11 is operated at an outlet temperature in the range of 1400 to l500 F. with a ratio of steam to naphtha, preferably being in the range of 0.4 to 0.8. Cracking zone 11 can be a thermal cracking unit or catalytic as desired. Catalysts that can be em- .ployed include acid catalysts, silica-alumina, molecular sieves, etc. Within cracking zone 11 the naphtha is converted to ethylene, propylene, butylenes, gasoline fraction and other materials. The effluent comprising the products of Ithermal cracking is removed from zone 11 by way of line 13 and passed to separation zone 14.

Separation zone 14 can comprise a plurality of separation units including fractionation, solvent extraction or any other separation known in .the prior art which would be chosen by one skilled in the art. A fuel gas stream is removed from zone 14 by way of line 15 for further use as desired. Such fuel gas can be conveniently used as a source of fuel in boilers, furnaces, etc. A hydrogen stream is removed from zone `14 by way of line 16 and passed to hydrotreater 17. Product ethylene is removed from zone 14 by way of line 18, product propylene is removed from zone 14 by way of line 19 .and a C4 fraction comprising butylenes and butadiene is removed from zone 14 by Way of line 20. A pyrolysis gasoline fraction is removed by way of line 21 and passed to hydrotreater 17. The pyrolysis gasoline fraction is hydrotreated in zone 17 to produce an effluent removed by line 22 comprising lighter hydrocarbons, primarily amylenes, which are passed to fractionation zone 23.

The hydrotreating zone 17 can be any of those known in the prior art which would be chosen by one skilled in the art. Typically, this zone is operated at a temperature in the range of 300 to 400 F., preferably in the presence of a catalyst such as sulded nickel catalyst. Fractionation zone 23 is operated so as to remove an amylenes fraction by way of line 24 and a fuel oil fraction by way of line 25 which is passed to further use as desired.-

An ethane fraction is removed from zone 14 by way of line 26 and passed to cracking zone 27 for conversion to ethylene, hydrogen and other products. Steam is introduced into the ethane stream produced in the cracking zone 27 .by way of line 28. Steam is preferably introduced near the entrance Iof the ethane stream into the cracking zone. The steam to ethane ratio will ordinarily be in the range of 0.4 to 0.6 and the cracking temperature employed will usually be about 1400 to 1600" F. The thermal cracking of ethane produces large quantities of hydrogen which ow along with the partially cracked ethane stream.

/The amylenes stream removed from fractionation zone 23 by line 24 is passed through heater 29 and line 30 land introduced into cracking zone 27 at a point downstream from the entrance of the ethane feed to cracking zone 27. The presence of hydrogen produced as part of the product of the ethane cracking assists in the cracking of the .amylenes introduced downstream in the partially cracked ethane stream whereby less dienes and more propylene and ethylene are produced. The operating temperatures for the first portion of the cracking zone for converting ethane and the operating temperatures for the second por-tion for the further conversion of ethane combined with the conversion of amylenes are set forth above. Cracking effluent comprising light hydrocarbons, as well as heavier hydrocarbons, hydrogen, etc., is removed from zone 27 by way of line 3.1 and line 32 and passed to separation zone 14 along with the effluent from naphtha cracking zone 11 in line 13. The combined effluents are separated as described above.

An aromatic-containing fraction is removed from zone 23 by way of line 33 and passed to aromatic extraction separation zone 34. The fraction admitted to zone 34 for separation can be contacted with a suitable solvent such as sulfolane, various glycols, etc., to remove as aromatic products therefrom including benzene, toluene, and xylene by way of lines 3'5, 36 and 37, respectively. A rafiinatc phase comprising parainic hydrocarbons is removed from zone 34 by way of line 38 and passed along with steam introduced by line 39 into cracking zone' 40; the effluent from zone 40 comprising light hydrocarbons is passed by way of line 41 and combined with lines 32 and 13 for introduction into separation zone 14 for separation of respective products as discussed above.

The hydrotreating process 1-7 preferably comprises a two-step operation in which the pyrolysis gasoline 21 is treated to convert at least a portion of the diolelins therein to linear and cyclic olefns, the fuel oil constituents being fractionated from the product. In the first step, oil pyrolysis gasoline 21 and hydrogen 16 are passed over a conventional catalyst known in the art, for example, cobalt and molybdenum type catalysts, to convert dioleiins to monooleiins. The reaction products are fractionated to yield fuel oil 25 and amylenes concentrate 24 and an aromatics-containing fraction 33.

An advantage of the instant invention by combining the ethane and amylenes cracking as described above is that more valuable products are produced than when ethane and amylenes are cracked separately.

The following example illustrates the advantages of the instant invention by combining the cracking of ethane and amylenes. The example sets forth reaction conditions, flow rates, etc. for separate cracking of amylenes and ethane as well as the combined cracking of ethane and amylenes, together with a comparison of yields after hydrotreating acetylenes in the fraction.

EXAMPLE (I) Separate cracking of ethane and of amylenes Pentene-2 cracking coil:

Radiant coil, inlet 1100 F. and 40 p.s.i.g.

Charge is `6030 pounds/hr. of pentene-Z plus 3020 pounds per hour steam.

Coil has 11 tubes of 4 inch inside diameter and 24 feet long (equivalent).

Residence time is 0.56 seconds.

Outlet is 1550 F. and 15 p.s.i.g. Ethane cracking coil:

Radiant coil, inlet 1100 F. and 45 p.s.i.g.

-Charge is 7400 pounds per hr. of ethane plus 2200 pounds per hour steam.

Coil has 30 tubes of 4.5 inch inside diameter and 35 feet long (equivalent).

Residence time is 2.1 seconds.

Outlet is l550 F. and 15 p.s.i.g.

(II) Combined cracking of ethane and amylenes Precracking coil, pentene-2:

Radiant section, inlet 1100 F. and 40 p.s.i.g. Charge is 4220 pounds/hr. pentene-2 plus 2110 pounds/ hr. steam.

Coil has 6 tubes of 3 inch inside diameter and 32 feet long (equivalent).

Residence time is 0.45 seconds. Outlet is 1500 F. at 21 p.s.i.g. Precracking coil-ethane:

' Radiant section, inlet 1100 F. at 40 p.s.i.g.

Charge is 4220 pounds/hr. ethane plus 1265 pounds/ hr. steam Coil has 25 tubes of 3.5 inch inside diameter and 32 feet long (equivalent).

Residence time is 2.0 seconds. Outlet is l500 F. at 2l p.s.i.g. Final cracking coil-mixture:

Inlet 1500 F. at 20 p.s.i.g. Charge is 8440 pounds/hr. hydrocarbon mixture plus 3375 pounds/hr. steam.

Coil has 2 tubes of 4 inch inside diameter and 38 feet long (equivalent).

Residence time is 0.1 seconds. Outlet is l550 F. at l5 p.s.i.g.

(III) COMPARISON OF YIELDS AFTER HYDROTREATING ACETYLENES IN THE FRACTIONS [Based on pounds of each of ethane and pontene-Z charged] Separate cracking Combined cracking C5 coil CZ coil Total total Hydrogen 0. 7 3. 2 3. 9 3. 5 Methane 14. 0 5. 8 19. 8 20. 5 Ethylene 16.6 45. 1 61.7 1 66. 4 3.9 41. 7 45. 6 43.8 9. 2 1. 5 10. 7 l 12. 9 0. 2 0. 6 0. 8 0. 9 29. 0 0. 7 29. 7 2 24. 6 4. G 0. 7 5. 3 5. 5 0. 9 0. 9 0. 8 20. 9 0.7 21. 6 2 21. 1

Pounds 100. 0 100. 0 200. 0 200. 0

l More. 2 Less.

It can be seen that the process of this invention results in substantial improvements in feed utilization, product distribution, reactor life, and process control due to 'the substantially reduced variation in residence times, i.e., the reduced rate of deposit formation and reactor volume occlusion.

Although the concept of this invention has been illustrated by examples employing thermal cracking procedures, it can be applied equally well to any cracking process, thermal or catalytic, where it is desirable to diminish the tendency of one or more reactant to form higher molecular weight derivatives thereof by the judicious combination of a series of cracking steps.

I claim:

1. In a hydrocarbon cracking process for cracking dissimilar hydrocarbon fractions derived from a hydrocarbon feed comprising paraliinic hydrocarbons and olefin hydrocarbons wherein a first fraction of said hydrocarbon comprising olefinic hydrocarbons which tend to form polyenes and higher molecular weight polymeric derivatives thereof under conditions to produce desired olefin products from each fraction while minimizing the production of undesirable polyenes and polymeric byproducts, the improvement which comprises cracking a second fraction of the hydrocarbon feed comprising paraffinic hydrocarbons which are less susceptible to the formation of higher molecular weight derivatives thereof in a first portion of a cracking zone to convert at least a portion thereof to hydrogen and desired olefin, and cracking said first fraction of said hydrocarbon feed in a second portion of said zone downstream of said first portion of said zone in the presence of said hydrogen abstracted from said second hydrocarbon fraction, thereby reducing the degree to wlnch higher molecular Weight polyenes and polymeric by-products are produced from the second portion of the cracking zone.

2. The process of claim 1 wherein said second fraction of the hydrocarbon comprises ethane and said first fraction comprises amylenes and said fractions are recovered from the effluent from a naphtha cracking.

3. The process of claim 1 wherein said second fraction of the hydrocarbon feed comprising ethane is substantially cracked prior to introduction of said first fraction into the partially cracked ethane so as to minimize the production of polyenes and other undesirable products.

4. The method of claim 1 wherein the degree of cracking of said first fraction is controlled by adjusting the ratio of said first fraction of said hydrocarbon feed to said second fraction of said hydrocarbon feed.

5. The method of claim 1 wherein said first fraction of said hydrocarbon feed is an amylenes fraction and said second fraction is ethane.

6. The process of claim 1 wherein said second fraction contains parafinic hydrocarbons having from 2-5 carbon atoms per molecule and said first fraction contains normal and branched chain olefin hydrocarbons having from 3-8 carbon atoms per molecule.

8 7. The process of claim 1 wherein said second fraction References Cited is ethane and said rst fraction is an amylenes fraction, T D ES PATENT both of which are separated from the eiuent of a naphtha UNI E STAT S treating and separated into a parainic raffinate stream PAUL M COUGHLAN JK Primary Examiner which is cracked and the effluent of this cracking is cornbined with the efuent from the naphtha cracking, and 10 C E' SPRESSER JR" Asslstant Exammer the combined ethane and amylenes Cracking and then U S, Cl. X.R. passed to a common separation. 208-130 

